The present invention relates to a production of a fuel cell separator, and more particularly to a mold capable of achieving a high dimensional accuracy in the molding process. Fuel cells of the type where a fuel gas and an oxidant gas are used, particularly, solid polymer type fuel cells have a structure in which an ion coductive solid electrolyte membrane is sandwiched between an anode and a cathode which are gas diffusion electrodes carrying a catalyst thereon, the sandwich being further sandwiched between separators. The separator on the anode side supplies hydrogen gas as the fuel gas to the anode, and the separtor on the cathode side supplies oxygen gas as the oxidant gas to the cathode.
FIG. 4 is a diagram illustrating such a fuel cell separator. As shown in the figure, a separator 1 has a planar surface with a narrow channel 1a provided thereon. In order to increase the contact area between the gas diffusion electrode and the gas, the channel 1a is meandering with a small pitch on the whole surface of the separator substantially across the entire surface of the separator. The channel 1a may be provided either on both sides of the separator, as illustrated in FIG. 4b, or on only one side thereof.
Such a separator 1 requires a high accuracy in it""s thickness. This is because the separator contacts with the anode or cathode and electricity is conducted therebetween, so that if the surface accuracy is poor, the contact area therebetween decreases, thereby lowering the conductivity. Moreover, if the surface accuracy is rough, a gap may occur between the separator and the anode or cathode so that a force is imparted in such a direction as to squash the gap, thereby cracking the separator. In other words, as the surface accuracy increases, the contact resistance decreases and the separator is less likely to crack, thus improving the performance of the fuel cell.
A conventional way of producing a fuel cell separator is as follows. First, as the starting material, a material compound is produced by, for example, mixing 25 parts by weight of a phenol resin to 100 parts by weight of a flake graphite (average particle diameter: 30 xcexcm), and the material compound is processed into a powder form. It is sometimes processed into a granular form or a pellet form, as well as a powder form.
As shown in FIG. 5a, the mold consists of a lower mold 2, a middle mold 3 and an upper mold 4. A cavity 5 for forming the fuel cell separator 1 therein is defined among the three molds. Specifically, the channel la for the reverse surface of the fuel cell separator 1 is provided on the lower mold 4; the middle mold 3 has an aperture which corresponds to the outer shape of the fuel cell separator 1; the channel la for the front surface of the fuel cell separator 1 is provided on the upper mold 2; and the distance between the upper and lower molds corresponds to the thickness of the separator.
The lower mold 4 and the middle mold 3 are attached to the fixed bed side of a press machine, and the upper mold is attached to the upper side of the press machine which can be raised and lowered.
The upper mold 2 is retracted by being raised up, and then the above-described powder form material is charged into the middle mold 3. The charging amount is adjusted to a weight which is slightly in excess of the weight of the produced fuel cell separator. This is to ensure that the material is sufficiently charged into every corner of the cavity 5. After the material is charged, the upper mold 2 is lowered to compress the powder form material, and the mold is heated by a heater (not shown) to about 432 K. The separator material is melted by the heat application, and is advanced into every corner of the cavity 5 by the applied pressure (about 2 KPa). Some gas vent holes (not shown) for communicating the cavity 5 to the outside is provided at appropriate locations of the mold to externally discharge the air which is originally contained in the cavity 5 and a gas which is generated from the material, thereby ensuring that no void is produced in the fuel cell separator. A vacuum pump may be used for the gas ventilation.
However, a mold as described above has the following problem. Since an amount of material that is slightly in excess of the amount sufficient to fill up the cavity is charged into the mold, there is some material which cannot be accommodated within the cavity 5 when the material is melted by the pressure and heat application. This excess of material moves up along the gap between the middle mold 3 and the lower mold 4 and then into the gap between the upper surface of the middle mold and the lower surface of the upper mold, where it is cooled and cured. This run-out portion is commonly called xe2x80x9cflashxe2x80x9d 6.
It is not problematic if the flash 6 is formed with a uniform thickness between the middle mold 3 and the upper mold 2. Typically, however, the flash 6 has an uneven distribution, e.g., it occurs in a single location as illustrated in the figure. As a result, the upper mold 2 is slanted as illustrated in FIG. 5b, thereby causing a 0.2 mm or greater difference or unevenness in the thickness of the produced fuel cell separator 1.
The present invention has been made in view of the above-described circumstances, and has an object of providing a mold for a fuel cell separator with which a flash, even if it occurs, does not cause a thickness unevenness.
In order to achieve the above-described object, the present invention provides a mold for producing a fuel cell separator, comprising a plurality of molds, a fuel cell molding cavity for separator being defined among the molds, wherein at least one of the molds is provided with a flash reservoir cavity which is communicated with the cavity.
The flash reservoir cavity may be provided by digging on a surface of one mold along which the mold contacts with another mold, so that the flash reservoir cavity extends to surround the fuel cell molding cavity for separator; the surface on which the flash cavity is provided by digging may closely contact the other mold, thereby defining the fuel cell molding cavity for separator; a sealing member may be provided external to the flash reservoir cavity for providing sealing; and the mold may be for use with a press machine.